The studies presented here investigate the stability of natural ecosystems, either in response to perturbations such as species loss, under the consideration of structural implications and species body masses. In a field survey I was experimentally excluding the predators of the herbivorous beetle Chrysomela aeneicollis. This perturbation altered the structure of the studied food web and simulated species loss at higher trophic levels. High predator diversity suppressed herbivores and consequently released plants from top-down pressure (trophic cascade). With a full-factorial design of predator removal, I could distinguish between the effects of diversity loss due to both additive effects of predators and predator compensation. Pair-wise predator-prey interaction strengths and larval survivorship of the beetles over time varied with predator diversity and the identity of co-existing predators, naimly their phenology. Subsequent theoretical model simulations seek to explain complex dependencies within food webs. A bioenergetic dynamically consumer-resource model presents a mechanistic explanation for why predator-prey body-mass ratios may be critically important for complex food-web stability. Simulations show that only certain combinations of body-mass ratios between three species in a food chain allow their stable co-existence. This 'stability domain' is restricted by bottom-up energy availability towards low and enrichment-driven dynamics towards high body-mass ratios. Consistently, more than 97% of three-species food chains across five natural food webs exhibit body-mass ratios within this 'stability domain'. Random re-wiring analyses of the food webs demonstrate that allometric link-degree distributions in natural food webs are critically important. They hold that the numbers of predators per species decreases whereas the number of prey per species increases with species’ body masses. Food-web stability emerges from these simple allometric link-degree distributions that are caused by physical constraints on predator-prey interactions. Food-web stability, however, is critically dependent on species loss. In a bioenergetic model approach I simulated species loss on data derived from nine empirically sampled food webs. Food-web robustness after species removal was measured depending on topological food-web parameters (e.g. diversity, number of basal species) and species traits (e.g. species body masses). The robustness of ecological networks after species loss is negatively related with network diversity, but positively correlated with the number of basal species and the average trophic level. Food-web robustness was higher when the species removed had small body sizes and high trophic levels. Early food-web models assumed the number of links per species to be scale independent, resulting in a decreasing connectance with increasing species number. However, other studies on new data showed these assumptions to be unrealistic and claimed "constant connectance" in food webs. I analyze existing relationships between diversity and complexity of natural food webs and discuss explanations for the meanwhile more broadly accepted scale dependence of complexity. I hypothesise that for example a decrease in connectance with increasing food-web complexity may be reasoned e.g. due to the difficulty to find weak links in larger systems (sampling effect). Further, an increase in habitat complexity might be dependent on an increase of specific sub-habitats, where predator and possible prey species are less likely to interact. Additional to the reviewing and discussion of possible mechanisms on scale dependence of complexity, the study includes own data analyses on one of the largest and best sampled empirical data sets available. These analyses reinvestigate common measures of bio-complexity and found a scale dependent behaviour of most food-web properties. Together, the experimental and theoretical work presented here contributes on the understanding on the dynamical processes between interacting species in ecosystems.